1. Field of the Invention
This invention relates generally to nuclear fuel handling apparatus and, more particularly, to an improved channel box removing apparatus which is adapted for use in removing the channel box from a nuclear fuel assembly placed on a fuel rack in a spent fuel storage pool in a nuclear power plant. This invention is particularly useful for nuclear fuel assemblies for a boiling water type nuclear reactor.
2. Background
Conventional designs of fuel assemblies for use in a boiling water type nuclear reactor include a multiplicity of fuel rods held in an organized array by spacers spaced along the fuel assembly length. As illustrated in FIG. 1 of the accompanying drawings, such a fuel assembly 10 includes a tubular channel box 12 which is generally square in cross section having a fuel assembly proper 14 mounted therein in a telescopic relationship to each other. The channel box 12 serves to promote contact of the fuel rods with primary coolant flow, facilitate guidance of the fuel rods in the reactor and protect the fuel assembly during transfer to and from the spent fuel storage pool 41 (FIG. 3). The channel box 12 is fastened to the fuel assembly proper 14 by means of fastener bolts 16.
A plurality, e.g., about 760 fuel assemblies are used in a light water reactor of a million KW class, and usually about 1/4 of the 760 assemblies, i.e., about 200 assemblies are replaced once a year during a refuelling operation.
During refuelling and other operations such as periodically performed inspections, the channel boxes 12 are removed from the associated fuel assemblies 10. FIG. 2 shows a conventional tool used to remove such channel boxes 12, which comprises a suspension ring-like member 22 adapted for suspension from a crane (not shown), an operating rod 24, and a channel box grappling section 26 mounted to the lower end of the operating rod 24. The channel box grappling section 26 includes a pair of rigid guide members 28 extending downwardly at opposite sides thereof to slidably receive therein the outer surfaces of a pair of diagonally opposed corners of the channel box 12, a pair of releasable hooks 30 pivotally mounted at their upper ends on the grappling section and adapted to engage the undersurfaces of associated clips (not shown) provided on the top of the channel box 12, and a control mechanism 32 for controling the movement of the releasable hooks 30 in response to a manually operated knob 34 provided on the top end of the operating rod 24. Removal of the channel box from the fuel assembly using such a removing apparatus is conventionally carried out during a refuelling operation within a spent fuel storage pool. Such removal of the channel box will be described hereinafter with reference to FIG. 3 which is a perspective view of the spent fuel storage pool.
(1) About 200 spent fuel assemblies previously transferred to a fuel rack 40 within the spent fuel storage pool 41 are lifted one by one by a main hoist 42 installed on a refuelling platform car 44 for transfer to a preparation machine 46.
(2) At the preparation machine 46, the fastener bolts threaded through the clips of each channel box to secure it and the fuel assembly proper together are first removed using a bolt wrench (not shown).
(3) Then, the removing apparatus 26 suspended from a jib crane 50 provided near the preparation machine 46 is lowered toward the fuel assembly and actuated to bring the releasable hooks 30 into engagement with the associated clips 86 of the channel box 12 to thereby hold the channel box 12 in a predetermined position. With the channel box 12 thus held, the fuel assembly proper 14 is lowered by the preparation machine 46 until it is fully removed from the channel box 12. The channel box 12 thus removed is transferred by the jib crane 50 to a channel box rack 52.
(4) Following the step (3), the fuel assembly proper 14 is transferred by the main hoist 42 from the preparation machine 46 back to the fuel rack 40 for temporary storage.
(5) When the channel box rack 52 becomes fully occupied by channel boxes transferred thereto, additional channel boxes 12 are transferred by a channel box transferring machine (not shown) attached to the lower end of the jib crane cable 50 to a waiting rack 54. When this waiting rack also becomes fully occupied, further channel boxes 12 are transferred again to the fuel rack 40 by a channel box transferring machine (not shown) which is connected to the lower end of a cable of an auxiliary hoist 56. Only flawless channel boxes placed on the fuel rack 40 are later reused.
Generally, the conventional channel box removing apparatus such as one generally designated at 26 has the limitation that it can be used only with such a preparation machine 46 which is capable of removing fuel assemblies, one at a time. Specifically, numerous fuel assemblies 10 are compactly placed on the fuel rack 40, leaving almost no space between the assemblies, as shown in FIG. 4. And, a framework 60 of the fuel rack 40 has a height slightly less than the upper end of each channel box 12. Accordingly, removal of the channel boxes from the fuel assemblies placed on the fuel rack 40 is not possible with the use of the conventional removal apparatus 26, since the guide members 54 cannot be brought into an operative position around the associated corners of the channel box 12 where the releasable hooks 30 can be operated into engagement with the associated clips 62 even if the removal apparatus is of a pneumatic drive type. Thus, it has been common practice to transfer fuel assemblies to the preparation machine 46, one at a time, where the channel box 12 is removed by lifting it from the fuel assembly proper 14. However, this causes inconveniences as set forth below:
(a) Separation of the channel box 12 from the fuel assembly proper 14 at the preparation machine 46 and individual transfer thereof, as described above, require each channel box 12 to be lifted and lowered many times by different hoists or cranes. Specifically, the channel box 12 is transferred by the jib crane 50 having a small radius of transfer to the channel box rack 52 or the waiting rack 54 for temporary storage, while the associated fuel assembly proper 14 is transferred by the main hoist 42 to the fuel rack 40. The channel box 12 is then transferred by the auxiliary hoist 56 to the fuel rack 40. Thus, many transfer steps are involved in an overall channel box removal operation, increasing the possibility of causing damage to channel boxes, prolonging the channel box removal time, and increasing the manpower required and the level of exposure for the operators.
(b) Also, there is a likelihood of the separated fuel assembly proper 14 being damaged during transfer to the fuel rack 40 because of lack of protection provided by the channel box. It has been proposed to use a pneumatic drive to control the releasable hooks of the removing apparatus rather than manually control the knob 34 and also to place a pedestal, called "channel storage adapter" (not shown), in the fuel rack 40 to raise the fuel assembly so as to expose the upper portion of the channel box 12 above the framework 60 so that the removal apparatus 26 can be brought into an operative position with the channel box for transfer from the preparation machine 46. However, the use of such pedestals complicates and prolongs the removal operation and also makes it more difficult to place the guide members 28 of the removal apparatus 26 on the associated corners of the channel box 12 because the fuel rack 40 is located fairly distant from an operating deck.